1. Field of the Invention
The present invention relates to projection optical apparatus for forming desired images, such as, exposure apparatus used in the fabrication of integrated circuits, and more particularly to improvements in the correction control of changes in the imaging characteristic of the apparatus.
2. Description of the Prior Art
One of the important optical characteristics of the projection optical apparatus, e.g., reduction projection exposure apparatus is the matching precision and a magnification error of the projection optical system is an important one of various factors which affect the matching precision.
In recent years, the scale of integration for integrated circuits has been improved with the resulting tendency toward the realization of finer patterns and consequently there has been an increasing demand for improvement in the matching precision. Thus, there has been a very high degree of necessity for maintaining the projection magnification at a desired value.
In this connection, the magnification of the projection optical apparatus varies in the vicinity of the desired magnification due to such causes as small temperature changes of the apparatus, small barometric changes and temperature changes of the atmosphere in the clean room in which the apparatus is installed or the projection of an energy beam to the projection optical system.
As a result, reduction projection exposure apparatus of the type employing a magnification correcting mechanism for effecting a fine adjustment of the magnification of the projection optical system and attaining the required magnification have recently been proposed. Such correction mechanisms have been so designed that as, for example, the spacing between a reticle and the projection lens is varied, the lens spacing in the projection lens is varied or the pressure in the suitable air chamber of the projection lens is adjusted as disclosed in U.S. patent application Ser. No. 656,777 filed on Oct. 1, 1984.
On the other hand, the focus of the exposure apparatus is also changed due to the similar reasons as the previously mentioned variation factors relating to the magnification. Thus, exposure apparatus employing focus correcting mechanisms have also been proposed.
Then, of these imaging characteristic changing factors the accumulation of heat due to the projection of an energy beam to the projection optical system is a heat diffusion phenomenon having a specific time constant. In the conventional exposure apparatus, the numerical apertures of their illuminating systems are generally constant in many cases. Thus, the manner in which the energy beam is incident to the projection optical system is fixed and the time constant of the heat diffusion is constant. As a result, the variation characteristics of the imaging characteristics, e.g., the magnification and focus, are constant and one and the same control method for their adjustment can be used.
In recent years, however, an exposure apparatus has been proposed in which the numerical aperture of the illuminating system is varied so as to obtain an improved resolving power for a specific pattern. The results of the experiments conducted on this exposure apparatus have shown that a change of the numerical aperture results in a change in the distribution of the illuminating light beam on the pupil of the projection optical system and therefore the time constant of the heat diffusion is also changed. Thus, there is a disadvantage that even if the previously mentioned control method relating to the constant time constant is applied, the imaging characteristic cannot be adjusted satisfactorily and it is impossible to deal with such change of the time constant.
Also, where the total amount of the incident energy beam to the projection optical system or the illuminance is measured on the image plane side of the projection optical system and the imaging characteristic is corrected by using the measured value as a parameter, the change is considered as a simple change in the incident energy quantity from the correction control point of view. Generally, while a change of the numerical aperture of the illuminating system results in the corresponding change in the illuminance at the image plane, the distribution of the light beam on the pupil, i.e., the energy density in the vicinity of the pupil plane, is also changed as mentioned previously. Thus, it is expected that not only the previously mentioned time constant but also the coefficient term of a model formula or the like for specifying the variation of the optical characteristic are changed. Therefor, there is a disadvantage that if such changes of the coefficient are not taken into consideration, the correction control is effected inaccurately.